The spine is made of bony structures called vertebral bodies that are separated by soft tissue structures called intervertebral discs. The intervertebral disc is commonly referred to as a spinal disc. The spinal disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions between vertebral segments of the axial skeleton. The disc acts as a synchondral joint and allows some amount of flexion, extension, lateral bending, and axial rotation.
The normal disc is a mixed avascular structure including two vertebral end plates, annulus fibrosis and nucleus pulposus. The end plates are composed of thin cartilage overlying a layer of hard, cortical bone that attaches to the spongy cancellous bone of the adjacent vertebral body.
The discs are subjected to a variety of loads as the posture of an individual changes. Even when the effects of gravity are removed, however, the soft tissue connected to the spine generates a compressive force along the spine. Thus, even when the human body is supine, the compressive load on the lumbar disc is on the order of 300 Newtons (N).
A spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the annulus fibers are weakened or torn and the inner material of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal annular confines. The mass of a herniated or “slipped” nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle strength and control or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and dehydrates with subsequent loss in disc height. Consequently, the volume of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping plies of the annulus buckle and separate, either circumferential or radial annular tears may occur, potentially resulting in persistent and disabling back pain. Adjacent, ancillary facet joints will also be forced into an overriding position, which may cause additional back pain.
When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity. Recently, efforts have been directed to replacing intervertebral discs which display degenerative disc disease. In one such procedure, the damaged intervertebral disc is replaced by a prosthetic disc.
One well known intervertebral prosthetic disc is produced by DePuy Spine, Inc. of Raynaham, Mass. and is sold under the trademark CHARITÉ®. This disc prosthesis includes two metal endplates and a center polyethylene core. The center core includes a superior spherical bearing surface and an inferior spherical bearing surface. The superior endplate includes a concave surface that fits upon and is congruent with the superior bearing surface of the core. The inferior endplate includes a concave surface that fits under and is congruent with the inferior bearing surface of the core.
During a CHARITÉ® artificial disc replacement procedure, the damaged disc is typically removed via an anterior surgical approach and the end surfaces of the exposed vertebrae are cleared of debris. The vertebrae are spread apart and the metal endplates are positioned on the respective vertebra and tapped into place. The polyethylene core is then inserted between the endplates and the vertebrae are returned to their normal position. The pressure of the spinal column further seats the endplates into the vertebral bones and secures the core in place.
While the sequential implantation of components is effective, the amount of time required to position three separate components as opposed to implanting a single unit increases the duration of the procedure. Additionally, the increased number of steps increases the risk of the procedure.
In response to the foregoing limitations, some instrumentation has been developed wherein a distraction instrument may also serve as an installation instrument. In particular, in addition to being configured to spread apart the two vertebrae, the instrument is also configured to slide the assembled artificial disc into place while the vertebrae remain separated. A central ramp is provided on the instrument to facilitate sliding of the implant between the vertebrae. Once the artificial disc is positioned, the installation instrument is decoupled from the artificial disc and removed.
Such instruments are very effective. Nonetheless, they do have various limitations. For example, because of the various functions performed with the instrument, the instruments are complicated in construction, resulting in increased costs. Additionally, as a particular instrument becomes more complicated, the potential for a mechanical failure increases. A further limitation is that the artificial disc is retained in such instruments using spring force which can be unreliable.
Accordingly, it would be advantageous to provide a tool for implanting an artificial disc or other spinal implant which does not rely upon a spring to maintain a secure hold upon the artificial disc or other spinal implant. It would also be advantageous if the tool could be used in combination with a distraction tool. It would be further advantageous if such features could be provided while allowing an artificial disc or other spinal implant to be implanted as a unit.